Surface Mount International Conference Proceedings


Author: Caroline Beelen
Company: CFT Centre for Mnfctrg Tech.
Date Published: 9/10/1996   Conference: Surface Mount International

Abstract: As a result of the trend towards portable consumer products ,low-cost miniaturization is becoming increasingly important. In this paper one of the items of low-cost miniaturization is discussed: the assembly of Flip Chips on standard Printed Circuit Boards (PCBs) in a standard reflow process. Flip Chip assembly with solder paste, and Flip Chip assembly, with tacky flux, are the two possibilities discussed. Using solder paste for Flip Chip assembly on standard PCB’s in a standard reflow process limits the number of additional process steps (namely, underfill application and curing) to a minimum. Other advantages that with solder paste the choice of bump material is he, and no special solder land finishes are required. Stencil printing of solder paste for Flip Chips necessitates the use of relatively thin stencils (50 -100 pm) with very small apetures to apply the small solder paste deposits required for bump pitches down to 250 pm. For a good print result, the size of the solder particles in the paste has to be reduced. The solder paste type must be such that flux residues do not interfere with the working of the underfiill material. Small solder particles influence the solder-balling behavior of the paste: as a result of the increased surface area of the solder, the number of solder balls increases drastically. This problem can be solved by soldering in a Nitrogen atmosphere. Another possibility of assembling Flip Chips on standard PCB’s in a standard reflow process is using tacky flux combined with eutectic solder bumps on the chips to provide the solder for the interconnections. The advantage here is that smaller bump pitches (down to about 160pm) can be processed. In this case, however, standard PCB’s can not be used. The requirements for the composition and size of the bumps are more stringent than for the solder paste process. Keywords: Flip Chip, reflow, miniaturization

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